Drafting

Drafting occurs when two or more moving vehicles or individuals align closely to reduce the overall effect of drag. Drafting is seen in competitive events such as auto racing, bicycle racing, speed-skating, and running.

Studies show that air flow over a single vehicle or individual in motion is characterized by a high-pressure region in front and a low-pressure region behind. The difference between these pressures creates a force, called drag, impeding motion. During drafting, as seen in the sketch below, a second vehicle or individual is closely aligned with another, and air flows over the pair nearly as if they were a single entity, thereby altering the pressure between them and reducing the drag each experiences. While race-car drivers use drafting to increase speed, non–motor sport competitors usually aim to reduce demands on their bodies while maintaining the same speed.

The energy required by animals to sustain life is derived from oxidation of ingested food. We often speak of food being burned in the body. This is an appropriate expression because experiments show that when food is burned with oxygen, approximately the same energy is released as when the food is oxidized in the body. Such an experimental device is the well-insulated, constant-volume calorimeter shown in Fig. below.

A carefully weighed food sample is placed in the chamber of the calorimeter together with oxygen (O2). The entire chamber is submerged in the calorimeter’s water bath. The chamber contents are then electrically ignited, fully oxidizing the food sample. The energy released during the reaction within the chamber results in an increase in calorimeter temperature. Using the measured temperature rise, the energy released can be calculated from an energy balance for the calorimeter as the system. This is reported as the calorie value of the food sample, usually in terms of kilo-calorie (kcal), which is the “Calorie” seen on food labels.

Ball and DetentA simple mechanical arrangement used to hold a moving part in a temporarily fixed position relative to another part. The ball slides within a bored cylinder, against the pressure of a spring, which pushes the ball against the detent, a hole of smaller diameter than the ball. When the hole is in line with the cylinder, the ball falls partially into the hole under spring pressure, holding the parts at that position. Additional force will push the ball back into its cylinder, compressing the spring, and allowing the parts to move.

Bearing The part of a machine within which a rotating or sliding shaft is held. In some bearing types, balls or rollers are used between the bearing surfaces to reduce rolling friction.

Bell crank A pivoting double lever used to change the direction of applied motion.

Boss A cylindrical projection, as on a casting or a forging. Usually provides a contact surface around a hole.

Broach To finish the inside of a hole to a shape other than round, as in a keyway. The tool for the process, which has serrated edges and is pushed or pulled through the hole to produce the required shape.

Burnish To smooth or polish by a rolling or sliding tool under pressure.

Bushing A smooth walled bearing (AKA a plain bearing). Also, a tool guide in a jig or fixture.

Cam A mechanical device consisting of an eccentric or multiply curved wheel mounted on a rotating shaft, used to produce variable or reciprocating motion in another engaged or contacted part (cam follower). Also, Camshaft.

Casting Any object made by pouring molten metal into a mold.

Chamfer A flat surface made by cutting off the edge or corner of an object (bevel).

Clevis A U-shaped piece with holes into which a link is inserted and through which a pin or bolt is run. It is used as a fastening device which allows rotational motion.

Collar A cylindrical feature on a part fitted on a shaft used to prevent sliding (axial) movement.

Collet A cone-shaped sleeve used for holding circular or rodlike pieces in a lathe or other machine.

Core To form the hollow part of a casting, using a solid form placed in the mold The solid form used in the coring process, often made of wood, sand, or metal.

Counterbore A cylindrical flat-bottomed hole, which enlarges the diameter of an existing pilot hole. The process used to create that feature.

CountersinkA conical depression added to an existing hole to accommodate and the conic head of a fastener recessing it below the surface of a face.

Coupling A device used to connect two shafts together at their ends for the purpose of transmitting power. May be used to account for minor misalignment or for mitigating shock loads.

Die One of a pair of hardened metal plates or impressing or forming desired shape. Also, a tool for cutting external threads.

Face To machine a flat surface perpendicular to the axis of rotation of a piece.

Fillet A rounded surface filling the internal angle between two intersection surfaces. Also Rounds

Fit The class of contact between two machined surfaces, based upon their respective specified size tolerances (clearance, transitional, interference)

Fixture A device used to hold a workpiece while manufacturing operations are performed upon that workpiece.

Sheave A grooved wheel used to accommodate a belt for the transmission of power. Sometimes referred to as a pulley sheave.

Shim A thin strip of metal inserted between two surfaces to adjust for fit.

Shoulder A plane surface on a shaft, normal to the axis, produced by a change in diameter.

Spline A cylindrical pattern of keyways. May be external (L) or internal (R)

Spotface a round machine surface around a hole on a casting or forging, usually to provide a contact surface for a fastener or other mating component.

Standoffs A mounting designed to position objects a predetermined distance above or away from the surface upon which they are mounted.

Tap To cut internal machine threads in a hole, the tool used to create that feature.

Undercut A cut having inward sloping sides, to cut leaving an overhanging edge

Yoke A clamp or vise that holds a machine part in place or controls its movement or that holds two such parts together. A crosshead of relatively thick cross section, that secures two or more components so that they move together.

Rotary and reciprocating compressors are both components of gas transfer systems. They both have the same purpose–to bring a gas into the system, inhale exhaust, then repeat the process. They both do this by changing the pressure at certain points in order to force gas in and exhaust out.

Pistons

One key difference is that reciprocating compressors use pistons while rotary compressors do not. A reciprocating compressor has a piston move downwards, reducing pressure in its cylinder by creating a vacuum. This difference in pressure forces the cylinder door to open and bring gas in. When the cylinder goes back up, it increases pressure, thus forcing the gas back out. The up-and-down motion is called a reciprocating motion, hence the name.

Rollers

Rotary compressors, on the other hand, use rollers. They sit slightly off-center in a shaft, with one side always touching the wall. As they move at high speeds, they accomplish the same goal as the reciprocating compressors–one part of the shaft is always at a different pressure than the other, so gas can come in at the low pressure point and exit at the high pressure point.

Advantages and Disadvantages

Reciprocating compressors are marginally more efficient than rotary compressors, generally being able to compress the same amount of gas with between 5 and 10 percent less energy input. However, since this difference is so marginal, most small-to-medium level users are best off using a rotary compressor. Reciprocating compressors are more expensive and require more maintenance, so it is often not worth the extra cost and headache for such a small difference in efficiency.

Large users, however, are generally best-served by reciprocating compressors. These are users for whom 5 percent represents a substantial figure, often substantial enough to justify the added expense.

The laws of thermodynamics dictate energy behavior, for example, how and why heat, which is a form of energy, transfers between different objects. The first law of thermodynamics is the law of conservation of energy and matter. In essence, energy can neither be created nor destroyed; it can however be transformed from one form to another. The second law states that isolated systems gravitate towards thermodynamic equilibrium, also known as a state of maximum entropy, or disorder; it also states that heat energy will flow from an area of low temperature to an area of high temperature. These laws are observed regularly every day.

Melting Ice Cube

Every day, ice needs to be maintained at a temperature below the freezing point of water to remain solid. On hot summer days, however, people often take out a tray of ice to cool beverages. In the process, they witness the first and second laws of thermodynamics. For example, someone might put an ice cube into a glass of warm lemonade and then forget to drink the beverage. An hour or two later, they will notice that the ice has melted but the temperature of the lemonade has cooled. This is because the total amount of heat in the system has remained the same, but has just gravitated towards equilibrium, where both the former ice cube (now water) and the lemonade are the same temperature. This is, of course, not a completely closed system. The lemonade will eventually become warm again, as heat from the environment is transferred to the glass and its contents.

Sweating in a Crowded Room

The human body obeys the laws of thermodynamics. Consider the experience of being in a small crowded room with lots of other people. In all likelihood, you’ll start to feel very warm and will start sweating. This is the process your body uses to cool itself off. Heat from your body is transferred to the sweat. As the sweat absorbs more and more heat, it evaporates from your body, becoming more disordered and transferring heat to the air, which heats up the air temperature of the room. Many sweating people in a crowded room, “closed system,” will quickly heat things up. This is both the first and second laws of thermodynamics in action: No heat is lost; it is merely transferred, and approaches equilibrium with maximum entropy.

Taking a Bath

Consider a situation where a person takes a very long bath. Immediately during and after filling up the bathtub, the water is very hot — as high as 120 degrees Fahrenheit. The person will then turn off the water and submerge his body into it. Initially, the water feels comfortably warm, because the water’s temperature is higher than the person’s body temperature. After some time, however, some heat from the water will have transferred to the individual, and the two temperatures will meet. After a bit more time has passed, because this is not a closed system, the bath water will cool as heat is lost to the atmosphere. The person will cool as well, but not as much, since his internal homeostatic mechanisms help keep his temperature adequately elevated.

Flipping a Light Switch

We rely on electricity to turn on our lights. Electricity is a form of energy; it is, however, a secondary source. A primary source of energy must be converted into electricity before we can flip on the lights. For example, water energy can be harnessed by building a dam to hold back the water of a large lake. If we slowly release water through a small opening in the dam, we can use the driving pressure of the water to turn a turbine. The work of the turbine can be used to generate electricity with the help of a generator. The electricity is sent to our homes via power lines. The electricity was not created out of nothing; it is the result of transforming water energy from the lake into another energy form.

Details

Torque or Turning Force:It is the total amount of force which is required to create acceleration on moving substance.

Couple:Two forces those acts on equally,parallely & oppositely on two separate points of same material.

Moment:It is the amount of moving effect which is gained for action of turning force.

Stress:It is the force that can prevent equal & opposite force. That means, it is the preventing force. If one force acts on outside of a material, then a reactive force automatically acts to protest that force. The amount of reactive force per unit area is called stress. e.g. Tensile Stress, Compressive Stress, Thermal Stress.

Strain:If a force acts on a substance, then in that case if the substance would deform. Then the amount of deformation per unit length of that substance is called strain.

Spring:It is one type of device which is being distorted under certain amount of load & also can also go to its original face after the removal of that load.

Its function:

To store energy.

To absorb energy.

To control motion of two elements.

Stiffness:Load per unit deflection. The amount of load required to resist the deflection.

Specific Weight:Weight per unit volume of the fluid.

Specific Volume:Volume per unit mass of the fluid.

Specific Gravity:It is the ratio of specific weight of required substance to specific weight of pure water at 4 degree centigrade temperature.

Viscosity:Dynamic Viscosity:The amount of resistance of one layer of fluid over other layer of fluid.

Kinematic Viscosity:It is the ratio of dynamic viscosity to density.

Buoyancy:When a body is immersed in a liquid, it is lifted up by a force equal to weight of liquid displaced by the body. The tendency of liquid to lift up an immersed body is buoyancy. The upward thrust of liquid to lift up the body is called buoyancy force.

Draft tube:It attaches with reaction turbine . Its function is to reduce energy loss from reaction turbine & it also reduce pressure at outlet which is must blow the atmospheric pressure.

Thermodynamics Laws:Zeroth Law:If two body are in thermal equilibrium with a third body then these two body are also in thermal equilibrium with each other.

First Law of Thermodynamics:In a closed system, work deliver to the surrounding is directly proportonal to the heat taken from the surrounding.And also, In a closed system, work done on a system is directly proportonal to the heat deliver to the surrounding.

Second Law of Thermodynamics:It is impossible to make a system or an engine which can change 100 percent input energy to 100 percent output.

Entropy:It is a thermodynamic property.

ds = dq/T

where, ds = change of entropy, dq = change of heat, T = Temperature.

In adiabatic process, entropy can not change. Actually,lacking or mal-adroitness of tranfering energy of a system is entropy.

Calorific Value of fuel:It us the total amount of heat obtained from burning 1 kg solid or liquid fuel.

Boiler/Steam Generator:It is a clossed vessel which is made of steel. Its function is to transfer heat to water to generate steam.

Economizer:It is a part of boiler. Its function is to heat feed water which is supplied to boiler.

Super-heater:It is a part of boiler. Its function is to increase temperature of steam into boiler.

Air-Preheater:It is a part of boiler. Its funtion is to preheats the air to be supplied to furnace and it recover heat from exhaust gas.

Boiler Draught:It is an important term for boiler. It is the difference of pressure above and below the fire grate. This pressure difference have to maintain very carefully inside the boiler. It actually maintains the rate of steam generation. This depends on rate of fuel burning. Inside the boiler rate of fuel burning is maintained with rate of entry fresh air. If proper amount of fresh air never entered into the boiler, then proper amount of fuel inside the boiler never be burnt. So, proper fresh air enters into the boiler only by maintaining boiler draught.

Nozzle:Nozzle is a duct of varying cros-sectional area. Actually, it is a passage of varying cross-sectional area. It converts steam’s heat energy into mechanical energy. It is one type of pipe or tube that carrying liquid or gas.

Scavenging:It is the process of removing burnt gas from combustion chamber of engine cylinder.

Supercharging:Actually, power output of engine depends on what amount of air enter into the engine through intake manifold. Amount of entry air if increased, then must be engine speed will increased. Amount of air will be increased by increasing inlet air density. The process of increasing inlet air density is supercharging. The device which is used for supercharging is called supercharger. Supercharger is driven by a belt from engine crankshaft. It is installed in intake system.

Turbocharging:Turbocharging is similar to the supercharging. But in that case turbocharger is installed in exhaust system whereas supercharger is installed in intake system. Turbocharger is driven by force of exhaust gas. Generally, turbocharger is used for 2-stroke engine by utilizing exhaust energy of the engine, it recovers energy otherwise which would go waste.

Governor:Its function id to regulate mean speed of engine when there are variation in the load. If load incrases on the engine, then engine’s speed must decrease. In that case supply of working fluid have to increase. In the otherway, if load decrease on the engine, then engine’ speed must increase. In that case supply of working fluid have to decrease.Governor automatcally, controls the supply of working fluid to the engine with varying load condition.

Flywheel:It is the one of the main parts of the I.C. engine. Its main function id to store energy in the time of working stroke or expansion stroke. And, it releasesenergy to the crankshaft in the time of suction stroke, compression stroke & exhaust stroke. Because, engine has only one power producing stroke.

C.I. Engine:Cetane Number. Cetane number indicates ability of ignition of diesel fuel. That means, how much fast ignites diesel fuel.

Stoichiometric ratio:It is the chemically correct air-fuel ratio by volume. By which theoretically sufficient oxygen will be gotten to burn all combustible elements in fuel completely.

Heat Transfer:It is a science which deals with energy transfer between material bodies as a result of temperature difference.There are three way to heat transfer such as-ConductionConvectionRadiation

Thermal Conductivity:It is the quantity of heat flows between two parts of solid material by conduction. In this case following consideration will be important fact-

Time—— 1 sec

Area of that solid material——– 1 m²

Thickness of that solid material—— 1m

Temperature difference between two parts of that material—— 1k

Heat Exchanger:It is one type of device which can transfer heat from one fluid to another fluid. Example- Radiator, inter-cooler, preheater, condenser, boiler etc.

Refrigeration:It is the process of removing heat from a substance. Actually, extraction of heat from a body whose temperature is already below the temperature of its surroundings.

1 tonne of refrigeration:It is amount of refrigeration effect or cooling effect which is produced by uniform melting of 1 tonne ice in 24 hours from or at 0 degree centigrade or freezing 1 tonne water in 24 hours from or at 0 degree centigrade.

Humidification:It is the addition of moisture to the air without change dry bulb temperatur.

Dehumidification:It is the removal of moisture from the air without change dry bulb temperature.

Gear Train:Meshing of two or more gear. It can transmit power from one shaft to another shaft.